It is important to detect a fuel injection condition, such as a fuel-injection-start timing, a maximum-fuel-injection-rate-reach timing, a fuel injection quantity and the like in order to accurately control an output torque and an emission of an internal combustion engine. Conventionally, it is known that an actual fuel injection condition is detected by sensing a fuel pressure in a fuel injection system, which is varied due to a fuel injection. For example, JP-2008-144749A (US-2008-0228374A1) describes that an actual fuel-injection-start timing is detected by detecting a timing at which the fuel pressure in the fuel injection system starts to be decreased due to a start of the fuel injection and an actual maximum fuel injection rate is detected by detecting a fuel pressure drop (maximum fuel pressure drop).
A fuel pressure sensor disposed in a common rail hardly detects a variation in the fuel pressure with high accuracy because the fuel pressure variation due to the fuel injection is attenuated in the common rail. JP-2008-144749A and JP-2000-265892A describe that a fuel pressure sensor is disposed in a fuel injector to detect the variation in the fuel pressure before the variation is attenuated in the common rail.
The present inventors has studied a method of computing a timing at which the fuel injection rate becomes a maximum value and a timing at which the fuel injection rate starts to fall from the maximum value based on a pressure waveform detected by the pressure sensor disposed in a fuel injector, which method will be described hereinafter.
As shown in FIG. 19A, when a command signal for starting a fuel injection is outputted from an electronic control unit (ECU) at a fuel-injection-start command timing “Is”, a driving current pulse supplied from an electronic driver unit (EDU) to a fuel injector starts to rise at the fuel-injection-start command timing “Is”. When a command signal for ending a fuel injection is outputted from the ECU at a fuel-injection-end command timing “Ie”, the driving current pulse starts to fall at the fuel-injection-end command timing “Ie”. A detection pressure detected by the fuel pressure sensor varies as shown by a solid line “L1” in FIG. 19B.
It should be noted that the command signal for starting a fuel injection is referred to as a SFC-signal and the command signal for ending a fuel injection is referred to as an EFC-signal, hereinafter.
When the SFC-signal is outputted from the ECU at the fuel-injection-start command timing “Is” and a fuel injection rate (fuel injection quantity per unit time) increases, the detection pressure starts to decrease at a changing point “P3b” on the pressure waveform. Then, when the fuel injection rate becomes a maximum value, a decrease in the detection pressure ends at a changing point “P4b” on the pressure waveform.
It should be noted that since the fuel flows toward an injection port by its inertia even after a timing of the maximum fuel injection rate, the detection pressure starts to increase after the decrease in the detection pressure ends at the changing point “P4b”. 
Then, when the EFC-signal is outputted at the fuel-injection-end command timing “Ie” and the fuel injection rate starts to decrease, the detection pressure starts to increase steeply at a changing point “P7b” on the pressure waveform. Then, when the fuel injection ends and the fuel injection rate becomes zero, the increase in the detection pressure ends at a changing point “P8b” on the pressure waveform.
Timings “t31” and “t32” at which the changing points “P4b” and “P7b” respectively appears are detected as a maximum-fuel-injection-rate-reach timing and a fuel-injection-rate-decrease-start timing, respectively. It should be noted that the maximum-fuel-injection-rate-reach timing is a timing at which the fuel injection rate becomes a maximum value, which is referred to as MFIRR timing, hereinafter. The fuel-injection-rate-decrease-start timing is a timing at which the fuel injection rate starts to fall, which is referred to as FIRDS timing, hereinafter.
Specifically, as shown by a solid line M1 in FIG. 19C, differential values are computed with respect to every detection pressure. After the SFC-signal is outputted and the detection pressure starts to decrease, the differential value first becomes zero at a timing “t31”. This timing “t31” is detected as the MFIRR timing at which the changing point “P4b” appears. Further, after the changing point “P4b”, the differential value first exceeds a threshold TH at a timing “t32”. This timing “t32” is detected as the FIRDS timing at which the changing point “P7b” appears.
In a case that a multi-stage injection is performed during one combustion cycle, a pressure pulsation is generated on the pressure waveform due to an overlapping of an aftermath (refer to an encircled portion “A0” in FIG. 19B) of a previous waveform with a current waveform. Also, a pulsation is generated in a waveform of the differential value of the detection pressure. Thus, according to the above described computing method, the MFIRR timing and the FIRDS timing can not be accurately computed. Especially, in a case that a multi-stage injection is performed, when an interval between n-th injection and (n+1)th injection is short, an unstable pressure waveform of n-th fuel injection overlaps with the pressure waveform of (n+1)th fuel injection. The pulsations of the pressure waveform and the differential value become large and an erroneous detection of the MFIRR timing and the FIRDS timing may be caused.
Moreover, it is conceivable that noises overlapping on the pressure waveform may cause a disturbance of the pressure waveform. Thus, even in a case that single-stage injection is performed or the interval is long, the above mentioned erroneous detection may be performed.
The present invention is made in view of the above matters, and it is an object of the present invention to provide a fuel injection detecting device capable of detecting a maximum-fuel-injection-rate-reach (MFIRR) timing and/or a fuel-injection-rate-decrease-start (FIRDS) timing with high accuracy based on a pressure waveform detected by a fuel pressure sensor.
According to the present invention, a fuel injection detecting device detecting a fuel injection condition is applied to a fuel injection system in which a fuel injector injects a fuel accumulated in an accumulator. The fuel injection detecting device includes a fuel pressure sensor provided in a fuel passage fluidly connecting the accumulator and a fuel injection port of the fuel injector. The fuel pressure sensor detects a fuel pressure which varies due to a fuel injection from the fuel injection port. Further, the fuel injection detecting device includes a changing point computing means for computing a changing timing, which is at least one of a fuel-injection-rate-decrease-start timing and a maximum-fuel-injection-rate-reach timing, based on a falling waveform of the fuel pressure during a period in which the fuel pressure decreases due to a fuel injection rate increase and a rising waveform of the fuel pressure during a period in which the fuel pressure increases due to the fuel injection rate decrease.
The fuel-injection-rate-decrease-start timing represents a timing at which the fuel injection rate starts to fall from a maximum fuel injection rate. The maximum-fuel-injection-rate-reach timing represents a timing at which the fuel injection rate becomes the maximum fuel injection rate.
When a command signal for starting a fuel injection is outputted, a fuel injection rate (fuel injection quantity per a unit time) starts to increase and the detection pressure detected by the fuel sensor starts to increase. After that, when a command signal for ending a fuel injection is outputted, a fuel injection rate starts to decrease and the detection pressure detected by the fuel sensor starts to increase. A falling pressure waveform and a rising pressure waveform hardly receive disturbances and their shapes are stable. Further, the falling waveform and rising waveform have high correlationship with the fuel-injection-rate-decrease-start timing and the maximum-fuel-injection-rate-reach timing.
According to the present invention, since the changing timing is computed based on the falling waveform and the rising waveform, the changing timing can be accurately computed without receiving any disturbances.
According to another aspect of the present invention, a fuel injection detecting device includes
an intersection timing computing means for computing an intersection timing at which a first line expressed by the falling-modeling function and a second line expressed by the rising-modeling function intersect with each other;
an intersection pressure computing means for computing an intersection pressure at which a first line expressed by the falling-modeling function and a second line expressed by the rising-modeling function intersect with each other;
a reference pressure computing means for computing a reference pressure based on a fuel pressure right before the falling waveform is generated;
a determination means for determining whether a pressure difference between the reference pressure and the intersection pressure is greater than a predetermined value; and
a changing point computing means for computing both a maximum-fuel-injection-rate-reach timing at which an output of the falling-modeling function is the predetermined value and a fuel-injection-rate-decrease-start timing at which an output of the rising-modeling function is the predetermined value, in a case that the difference between the reference pressure and the intersection pressure is greater than the predetermined value.